<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <button id="btn">单击</button>
</body>
</html>
<script>
    var button=document.getElementById('btn')
    button.onclick=function(){
        alert('hellol!')
    }
</script>